Percolation via combined electrostatic and chemical doping in complex oxide films

TL;DR

This paper theoretically investigates how combined chemical and electrostatic doping induce percolation in complex oxide films, revealing mechanisms that depend on film thickness and impact magnetic properties, guiding experimental efforts.

Contribution

It introduces two mechanisms of doping-induced percolation in complex oxides and analyzes their dependence on film thickness and magnetic properties.

Findings

Bulk-assisted surface percolation reduces electrostatic charge needed.

Thin films can reach percolation with modest surface charge.

Magnetic clusters extend from surface, enhancing magnetization.

Abstract

Stimulated by experimental advances in electrolyte gating methods, we investigate theoretically percolation in thin films of inhomogenous complex oxides, such as La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), induced by a combination of bulk chemical and surface electrostatic doping. Using numerical and analytical methods, we identify two mechanisms that describe how bulk dopants reduce the amount of electrostatic surface charge required to reach percolation: (i) bulk-assisted surface percolation, and (ii) surface-assisted bulk percolation. We show that the critical surface charge strongly depends on the film thickness when the film is close to the chemical percolation threshold. In particular, thin films can be driven across the percolation transition by modest surface charge densities \emph{via} surface-assisted bulk percolation. If percolation is associated with the onset of ferromagnetism, as in LSCO, we further demonstrate that the presence of critical magnetic clusters extending from the film surface into the bulk results in considerable volume enhancement of the saturation magnetization, with pronounced experimental consequences. These results should significantly guide experimental work seeking to verify gate-induced percolation transitions in such materials.